In recent years, in the field of display devices, the market of 3D display devices and organic EL display devices has been expanding. As a demand for an increase in definition and image quality increases, Cu wiring materials of low resistance and oxide semiconductors which are high-mobility semiconductor materials are receiving much attention. The oxide semiconductors are expected to be about ten times higher in mobility than conventional a-Si. Copper is lower in resistivity than Al. The electrical resistivity of Al is 2.5×10−6 Ω·cm, whereas the electrical resistivity of Cu is low, 1.6×10−6 Ω·cm.
By the way, for a thin-film transistors (TFT) for use in display devices, it is necessary to form a passivation film (passivation layer) made of an insulating oxide such as SiNx, SiON, or AlOx on the channel region of the TFT in order to suppress the time-dependent deterioration of TFT properties. The passivation film is usually formed (deposited) by a plasma-enhanced CVD process, a sputtering process, or the like. An example of a method for forming an SiNx passivation film by a plasma CVD process is a method in which SiNx is formed by allowing a gas mixture of SiH4 and N2O to react in high-frequency plasma with an industrial frequency of 13.56 MHz and is deposited on an oxide semiconductor film. The following method has been proposed: a method for forming an Al2O3 passivation film by a reactive sputtering process using oxygen.
However, when a passivation film is formed, radicals or molecules sped up by plasma collide with the surface of an oxide semiconductor to apply plasma damage to an oxide semiconductor layer or hydrogen diffuses from the passivation film to make the oxide semiconductor layer conductive in some cases. Therefore, in order to prevent the reduction of TFT properties due to surface defects (damage) of the oxide semiconductor during the formation of the passivation film, for example, Non-patent Literature 1 proposes a method in which an oxide semiconductor layer is rendered non-conductive in such a way that the surface of an oxide semiconductor is excessively oxidized in advance by irradiating the oxide semiconductor surface with N2O plasma just before a passivation film is formed (plasma treatment prior to the formation of the passivation film).
In a display device including such an oxide semiconductor layer, plasma treatment (the formation of a film or the removal of a surface layer in a plasma environment) is performed in a step of forming a passivation layer or prior thereto. In this operation, the surface of a Cu wiring line used for source-drain electrodes is exposed to plasma containing oxygen atoms, thereby causing a problem that the surface of the Cu wiring line is oxidized. The surface oxidation of the Cu wiring line makes the adhesion of the Cu wiring line to a passivation film placed thereon insufficient; hence, a wiring failure such as the uplift of the passivation film may possibly occur and a problem such as an increase or variation in contact resistance with a transparent conductive film may possibly occur. If an oxide layer is formed by the surface oxidation of the Cu wiring line, then the surface roughness of the Cu wiring line increases and therefore the coverage of the passivation film becomes insufficient. This allows the entry of water from outside. Therefore, the primary role of the passivation film, that is, the effect of suppressing the time-dependent deterioration of TFT properties is not sufficiently obtained or failures due to the oxidation or corrosion of wiring lines may possibly occur.
Techniques for preventing the oxidation of Cu wiring lines used in integrated circuits or the like for semiconductor devices have been proposed as described in Patent Literatures 1 to 4 below.
Patent Literatures 1 and 2 each disclose a method in which an oxide film with excellent oxidation resistance is formed in such a way that Al or Si is diffusively concentrated near the surface of a wiring line by oxidizing a Cu alloy containing Al or Si. In particular, this method is as follows: a predetermined oxide film is formed by making use of annealing (heat treatment) at about 300° C. to 500° C. in the course of fabricating TFTs or by performing oxidative heat treatment at 500° C. or lower separately from annealing. Patent Literature 3 discloses a method in which an Al—Cu alloy film is formed so as to entirely cover the surface of a Cu wiring line. Patent Literature 4 discloses a technique in which the oxidation of a Cu wiring line by an etching agent or a photoresist stripper is suppressed in such a way that a copper compound layer made of copper phosphide, copper boride, copper bromide, or copper nitride on the surface of the Cu wiring line.